The Xenograft and Cell Line Core (Core B) will provide state of the art molecularly characterized human models representing childhood sarcoma histotypes to be studied in Projects 1-3. Patient derived xenografts (PDX) representing Ewing sarcoma (n=7). Osteosarcoma (n=10) and rhabdomyosarcoma (n=15) and cell lines (n=16) are available for study. Authenticity and validity of lines within the Core have been determined by periodic short tandem repeat (STR) analysis, and molecular profiling. The Core will provide subcutaneous, orthotopic and disseminated disease models as required for individual projects, as well as services listed: 1. Maintain and characterize human xenografts, and maintain frozen stocks of xenografts authenticated by short tandem repeat (STR) analysis. 2. Maintain and characterize cell lines by STR analysis for use in Projects 1-3. 3. To provide a facility for the propagation of xenografts of pediatric solid tumors, for use in all Projects. 4. Provide orthotopic and disseminated xenograft models for secondary testing. 5. Coordinate tumor transplantation and make available tumor-bearing mice for pharmacodynamic and pharmacokinetic studies as required. 6. Coordinate treatment and tissue collection. 7. Undertake drug evaluation studies using standardized protocols, and transfer data to the Biostatics Core. 8. Provide other services (e.g. blood and tissue collection, for PK/PD studies). 9. Assist in planning and experimental design in consultation with the biostatistics Core A. Although this is a new Core, the Core director has over 35 years experience developing and using human tumor xenograft models, and has published 50 papers reporting the evaluation of new agents using these models since 2007. The Core has provided services that have resulted in two publications since the AO submission.